Both. I think you lose a required level of effectiveness in the process if you rely entire on one aspect or the other.
As the manager, you need to decide what your priorities are, what your needs are, and how to most effectively build your squad. You need to take into account your tactical plans, both short term and long term. You need to examine your current squad in comparison to your tactical plans and goals. The scouts will not do these things for you.
At the same time, simply looking at attributes won't tell you whether they have the potential to get better. It won't tell you about really important hidden attributes like injury proneness, consistency, big match performances, and adaptability. But don't take the "must buy" recommendation as meaning you MUST spend your money on this player. that's information to help you make an informed decision, not the decision itself. That has to remain with you, the manager.
You will get "must buy" recommendations for loads of high potential players, depending on what club you are and where you scout. But many will be of moderate uselessness to you. Why spend a chunk of money on a small, dribbly, creative False 9 type forward when you want and need a big strong type of lone striker for your tactic? Why buy another prospect at a position you already have some depth if you REALLY need a first team caliber player for a different position? Why buy an attacking-only lightweight winger when you want to play a defensively, conservative style? these are the aspects scouting doesn't really take into account. You can direct scouts what to look for in terms of player types, but if they are openly scouting or you ask them to scout a specific player, you won't get a report that says "really good player but they don't fit our current tactical outlook".
The real key, IMO, is understanding your squad, having plans, and knowing what you are looking for. If you haven't done that part, you haven't player taken on your role as manager.
An example - on a past FM, I was seeking a really specific type of player. A defensive minded midfielder in their late 20s. I wanted a purely defensive player who would be used in occasional matches where I needed that kind of steel in my spine, or more likely as a match closer, coming on as a sub to help shut up shop in the last 30 minutes or so. Technical attributes didn't matter besides tackling and marking. Mental attributes were key. So was personality - I wanted him to be able to mentor. But because he wouldn't be starting regularly, a high reputation (which would mean he would want a higher squad status) was a negative. Didn't want to spend a ton. Between getting a scout to look for this exact type of player and digging myself, I found a few options. Most of whom didn't get a passing recommendation from my scouts, because they were such limited players. But using my scouts was key to know their hidden attributes. If I listened to my scouts, I never would have bought the player I did, who worked out pretty well.